Taiji 太極 (literally
"great pole") is a Chinese cosmological term for the "Supreme
Ultimate" state of undifferentiated absolute and infinite potentiality,
contrasted with the Wuji 無極 "Without Ultimate". Taiji is best known in the West from
the name Taijiquan (or T'ai chi ch'uan) 太極拳 "Supreme Ultimate Fist"[citation needed].

Common English translations of the
cosmological Taiji are the "Supreme Ultimate" (Le Blanc 1985, Zhang
and Ryden 2002) or "Great Ultimate" (Chen 1989, Robinet 2008); but
other versions are the "Supreme Pole" (Needham and Ronan 1978),
"Great Absolute", or "Supreme Polarity" (Adler 1999).

Any philosophy that asserts two elements
such as the yin-yang of Chinese philosophy will also look for a term to
reconcile the two, to ensure that both belong to the same sphere of discourse.
The term 'supreme ultimate' performs this role in the philosophy of the Book of
Changes. In the Song dynasty it became a metaphysical term on a par with the
Way. (2002:179)

The Way has attributes and evidence, but it
has no action and no form. It may be transmitted but cannot be received. It may
be apprehended but cannot be seen. From the root, from the stock, before there
was heaven or earth, for all eternity truly has it existed. It inspirits demons
and gods, gives birth to heaven and earth. It lies above the zenith but is not
high; it lies beneath the nadir but is not deep. It is prior to heaven and
earth, but is not ancient; it is senior to high antiquity, but it is not old.
(tr. Mair 1994:55)

Taiji also appears in the Xìcí 繫辭
"Appended Judgments" commentary to the Yijing, an early section
traditionally attributed to King Wen of Zhou and Duke of Zhou.

Therefore there is in the Changes the Great Primal Beginning. This
generates the two primary forces. The two primary forces generate the four
images. The four images generate the eight trigrams. The eight trigrams
determine good fortune and misfortune. Good fortune and misfortune create the great
field of action. (tr. Wilhelm and Baynes 1967:318-9)

The fundamental postulate is the
"great primal beginning" of all that exists, t'ai chi – in its
original meaning, the "ridgepole". Later Chinese philosophers devoted
much thought to this idea of a primal beginning. A still earlier beginning, wu
chi, was represented by the symbol of a circle. Under this conception, t'ai chi
was represented by the circle divided into the light and the dark, yang and
yin, Yin yang.svg. This symbol has also played a significant part in India and
Europe. However, speculations of a Gnostic-dualistic character are foreign to
the original thought of the I Ching; what it posits is simply the ridgepole,
the line. With this line, which in itself represents oneness, duality comes
into the world, for the line at the same time posits an above and a below, a
right and left, front and back – in a word, the world of the opposites.
(1967:lv)

Zhou's key terms Wuji and Taiji appear in
the opening line 自無極而太極, which Adler notes could also be translated "The Supreme
Polarity that is Non-Polar!".

Non-polar (wuji) and yet Supreme Polarity
(taiji)! The Supreme Polarity in activity generates yang; yet at the limit of
activity it is still. In stillness it generates yin; yet at the limit of stillness
it is also active. Activity and stillness alternate; each is the basis of the
other. In distinguishing yin and yang, the Two Modes are thereby established.
The alternation and combination of yang and yin generate water, fire, wood,
metal, and earth. With these five [phases of] qi harmoniously arranged, the
Four Seasons proceed through them. The Five Phases are simply yin and yang; yin
and yang are simply the Supreme Polarity; the Supreme Polarity is fundamentally
Non-polar. [Yet] in the generation of the Five Phases, each one has its nature.
(tr. Adler 1999:673-4)

Taiji is understood to be the highest
conceivable principle, that from which existence flows. This is very similar to
the Daoist idea "reversal is the movement of the Dao". The
"supreme ultimate" creates yang and yin: movement generates yang;
when its activity reaches its limit, it becomes tranquil. Through tranquility
the supreme ultimate generates yin. When tranquility has reached its limit,
there is a return to movement. Movement and tranquility, in alternation, become
each the source of the other. The distinction between the yin and yang is
determined and the two forms (that is, the yin and yang) stand revealed. By the
transformations of the yang and the union of the yin, the 5 elements (Qi) of
water, fire, wood, metal and earth are produced. These 5 Qi become diffused,
which creates harmony. Once there is harmony the 4 seasons can occur. Yin and
yang produced all things, and these in their turn produce and reproduce, this
makes these processes never ending. (Wu, 1986)